Group III nitride compound semiconductor device

ABSTRACT

Aluminum gallium nitride (Al x Ga 1−x N, 0&lt;x&lt;1) is employed as a substrate of a Group III nitride compound semiconductor device. In light-emitting diodes and laser diodes employing the substrate, crack generation is prevented, even when a thick cladding layer formed of aluminum gallium nitride (Al x Ga 1−x N, 0&lt;x&lt;1) is stacked on the substrate. The smaller the difference in Al compositional proportion between the substrate and an aluminum gallium nitride (Al x Ga 1−x N, 0&lt;x&lt;1) layer, the less likely the occurrence of crack generation.

This is a continuation Application of application Ser. No. 09/654,492filed Sep. 1, 2000, the entire contents of which is hereby incorporatedby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a Group III nitride compoundsemiconductor device and, more particularly, to a Group III nitridecompound semiconductor device which functions as a light-emittingdevice.

2. Background Art

A Group III nitride compound semiconductor is one type of semiconductorused among direct transition types of semiconductors, which have anemission spectrum widely ranging from ultraviolet to red. Thus, thesemiconductor is employed in light-emitting devices such aslight-emitting diodes (LEDs) or laser diodes (LDs). In a devicefabricated by stacking Group III nitride compound semiconductor layers,a sapphire substrate is typically employed as a substrate in vie(because) of its proximity in lattice constant. FIGS. 3 and 4 showstructures of the semiconductor devices.

FIG. 4 shows a conventional laser diode (LD) 950 employing Group IIIcompound semiconductors. In the LD, an AlN buffer layer 912, an n-GaNn-contact layer 913, an n-Al_(x)Ga_(1−x)N n-cladding layer 914, an n-GaNn-guide layer 915, an emission layer 916 formed, preferably of amulti-layer (multiple quantum well, MQW), a p-GaN p-guide layer 917, ap-Al_(x)Ga_(1−x)N p-cladding layer 918, and a p-GaN p-contact layer 919are formed on a sapphire substrate 911, in the order presented. Apositive electrode 920A is formed on the p-contact layer 919 while anegative electrode 920B, developed through etching, is formed on aportion of the n-contact layer 913.

However, the aforementioned conventional semiconductor has a drawback.Specifically, when a Group III nitride compound semiconductor is formedon a sapphire substrate, cracks are generated in a semiconductor layer,or a semiconductor layer bends, since elastic modulus and thermalexpansion coefficient of the sapphire substrate differ from those of theGroup III nitride compound semiconductor. Thus, the fabricated devicehas poor device characteristics. In addition, although the latticeconstant of the sapphire substrate is approximately equal to that of theGroup III nitride compound semiconductor, dislocations are readilygenerated due to misfit. Particularly, a cladding layer formed ofAl_(x)Ga_(1−x)N attains a higher elastic modulus as the compositionalproportion of Al (hereinafter simply referred to as “x”) increases.Therefore, cracks are readily generated in such a cladding layer duringa cooling process in production of a semiconductor device. As a result,the thickness of the cladding layer, which has a large compositionalproportion of Al is limited to a low value. Such limitation in thicknessis particularly detrimental to fabrication of laser diodes.

Employment of a sapphire substrate, which is an electric insulator,raises another limitation for fabricating semiconductor devices.Specifically, a positive electrode and a negative electrode must bedisposed on a semiconductor-formed surface of a sapphire substrate.

SUMMARY OF THE INVENTION

In view of the foregoing disadvantages, an object of the presentinvention is to provide a Group III nitride compound semiconductordevice in which generation of cracks in a semiconductor layer, bendingof a semiconductor layer, and generation of misfit-induced dislocationin a semiconductor layer are prevented. Another object of the inventionis to provide a Group III nitride compound semiconductor deviceemploying a conductive substrate through which electricity is passed.

Accordingly, the present invention is directed to a Group III nitridecompound semiconductor device comprising a substrate and one or moreGroup III nitride compound semiconductor layers formed on a firstsurface or first and second surface of the substrate, wherein aluminumgallium nitride (Al_(x)Ga_(1−x)N, 0<x<1) is employed as the substrate.

Preferably, among the Group III nitride compound semiconductor layersstacked on a first surface or first and second surfaces of a substrate,all layers, having a thickness of more than 10 nm, are independentlyformed of a compound represented by Al_(x)Ga_(1−x)N, wherein 0≦x≦1.

Preferably, each of the Group III nitride compound semiconductor layersstacked on a first surface or first and second surfaces of a substrateis formed of a compound represented by Al_(x)Ga_(1−x)N, wherein 0≦x≦1.

Preferably, a first layer of a Group III nitride compound semiconductorlayer stacked on a first surface or a first and second surface of asubstrate has a thickness of 1-20 μm, more preferably 2-20 μm.

A group III nitride compound semiconductor layer comprises binarycompounds such as AlN, GaN, and InN. A group III nitride compoundsemiconductor layer also comprises ternary compounds such asAl_(x)Ga_(1−x)N, Al_(x)In_(1−x)N, and Ga_(x)In_(1−x)N (0<x<1). And agroup III nitride compound semiconductor layer further comprisesquaternary compounds such as Al_(x)Ga_(y)In_(1−x−y)N (0≦x≦1, 0≦y≦1,0≦x+y≦1). In the present invention, unless otherwise specified, the term“Group III nitride compound semiconductors” encompasses Group IIInitride compound semiconductors per se and Group III nitride compoundsemiconductors doped with an impurity which causes the semiconductors tobecome either p- or n-conduction type semiconductors. Likewise, aluminumgallium nitride (Al_(x)Ga_(1−x)N, 0<x<1) also encompasses dopedsemiconductors.

BRIEF DESCRIPTION OF THE DRAWINGS

Various other objects, features, and many of the attendant advantages ofthe present invention will be readily appreciated as the same becomesbetter understood with reference to the following detailed descriptionof the preferred embodiments when considered in connection withaccompanying drawings, in which:

FIG. 1 is a schematic cross-sectional view of the structure of a GroupIII nitride compound semiconductor device according to a firstembodiment of the present invention;

FIG. 2 is a schematic cross-sectional view of the structure of a GroupIII nitride compound semiconductor device according to a secondembodiment of the present invention;

FIG. 3 is a schematic cross-sectional view of the structure of aconventional light-emitting diode comprising a Group III nitridecompound semiconductor; and

FIG. 4 is a schematic cross-sectional view of the structure of aconventional laser diode comprising a Group III nitride compoundsemiconductor.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A Group III nitride compound semiconductor device such as alight-emitting device is produced by doping an appropriate amount ofimpurity to a Group III nitride semiconductor component layerrepresented by Al_(x)Ga_(y)In_(1−x−y)N (0≦x≦1; 0≦y≦1; 0≦x+y≦1). Asdiscussed above with regard to the semiconductor layers, cracks arereadily generated in an aluminum gallium nitride (Al_(x)Ga_(1−x)N,0<x<1) layer when the layer is formed so as to have a large thicknesssince the sapphire substrate has an elastic modulus and thermalexpansion coefficient greatly different from those of the aluminumgallium nitride (Al_(x)Ga_(1−x)N, 0<x<1) layer. Thus, when a substratein which the ‘x’ is equal or nearly equal to the ‘x’ in an aluminumgallium nitride (Al_(x)Ga_(1−x)N, 0<x<1) layer formed in the device, thedifference in the elastic modulus and thermal expansion coefficientbetween the substrate and the semiconductor layer can be su pressed to aminimum level. Accordingly, even when a substrate has a thickness of 50μm or more, further 100 μm or more, or an aluminum gallium nitride(Al_(x)Ga_(1−x)N, 0<x<1) cladding layer is formed so as to have athickness of 1 μm or more, no cracks are generated, due to the smalldifference in the elastic modulus and thermal expansion coefficientbetween the substrate and a Group III nitride semiconductor componentlayer. The cladding layer preferably has a thickness of 1 μm or more inorder to ensure the crystallinity of a layer formed thereon. Inconnection with the upper limit, the thickness is preferably 20 μm orless, more preferably 2-10 μm, in view of the productivity of devices.

When each Group III nitride compound semiconductor layer is formed fromAl_(x)Ga_(1−x)N (0<x<1); such as, gallium nitride, aluminum galliumnitride, or aluminum nitride, the overall thickness of the stackedlayers is approximately 20 μm. In such a semiconductor, the formation ofcracks induce by the difference in the elastic modulus and thermalexpansion coefficient between he substrate and a Group III nitridesemiconductor component layer can be prevented. When an emission layerof a light-emitting device is formed of a multiple quantum well, a welllayer and a barrier layer, the emission layer may be sufficiently thinand does not have to be formed of Al_(x)Ga_(1−x)N (0<x<1). Such a modeis also included within the scope of the invention. The aforementionedGroup III nitride compound semiconductor devices are particularlypreferred as light-emitting devices, such as light-emitting diodes(LEDs) and laser diodes (LDs), which require a long service life.Specifically, in the production of an LD, when a thick cladding layer isformed from an aluminum gallium nitride Al_(x)Ga_(1−x)N (0<x<1) and the‘x’ of an aluminum gallium nitride Al_(x)Ga_(1−x)N (0<x<1) substratewhich ‘x’ is equal or nearly equal to ‘x’ in the cladding layer, therecan readily be prevented cracks, induced by difference in elasticmodulus and thermal expansion coefficient between he substrate and aGroup III nitride semiconductor component layer. When the ‘x’ of thecladding layer coincides with the ‘x’ of the substrate, the ‘x’ can beelevated and all semiconductor component layers can contain aluminum.Furthermore, when aluminum gallium nitride Al_(x)Ga_(1−x)N (0<x<1)having a desired electric conductivity is employed as a substrate, aGroup III nitride compound semiconductor light-emitting device isproduced in which electricity is passed via the aluminum gallium nitridesubstrate.

The Group III nitride compound semiconductor device of the presentinvention may also be adapted to power devices and light-emittingdevices other than LEDs and LDs. When a contact electrode is formed on asubstrate, a Si-doped GaN contact layer may be formed so as to lower thecontact resistance. Semiconductor component layers may also be formed ofa composition-graded layer; e.g., from GaN to Al_(x)Ga_(1−x)N.

The present invention will next be described in detail with reference tospecific embodiments, which should not be construed as limiting theinvention thereto.

First Embodiment

A light-emitting diode 100 having a structure shown in FIG. 1 wasproduced. The LED 100 contains an n-type Al_(0.07)Ga_(0.93)N substrate 1having a thickness of approximately 100 μm and an electron density of3×10¹⁷/cm³.

The following layers were formed on the n-type Al_(0.07)Ga_(0.93)Nsubstrate 1, in the order presented, an Si-doped Al_(0.07)Ga_(0.93)Nn-type cladding layer 2 having a thickness of approximately 0.5 μm andan electron density of 2×10¹⁸/cm³; a GaN active layer 3 having athickness of 50 nm; an Mg-doped Al_(0.07)Ga_(0.93)N p-type claddinglayer 4 having a thickness of approximately 0.5 μm and a hole density of5×10¹⁷/cm³. A metal electrode 5A was formed on the cladding layer 4, anda metal electrode 5B was formed on the entire backside of the n-typeAl_(0.07)Ga_(0.93)N substrate 1.

The metal electrode 5A, serving as a positive electrode, was formed fromgold (Au). Alternatively, the electrode may be formed from Au—Co alloy,Au—Ni alloy, Au—other metal alloy, or a multi-layer thereof. The metalelectrode 5B, serving as a negative electrode, was formed from aluminum(Al). Alternatively, the electrode may be formed from Al—V alloy, Al—Tialloy, Al-other metal alloy, or a multi-layer thereof.

A method for producing the light-emitting diode 100 shown in FIG. 1 willnext be described. In practice, the Al_(0.07)Ga_(0.93)N substrate 1 wasproduced by epitaxially growing Al_(0.07)Ga_(0.93)N on a silicon (Si)substrate employing a halide source and removing the silicon Sisubstrate. More specifically, a silicon (Si),substrate was placed in achamber equipped with a halogen source-supplier. The chamber wasevacuated, and nitrogen (N₂) was fed into the chamber. The atmospherewas heated to 1000° C., to thereby cause gallium (Ga) and aluminum (Al)to react with hydrogen chloride (HCl). The chloride formed was suppliedto the Si substrate, while ammonia (NH₃) was introduced to the chamber,thereby forming Al_(0.07)Ga_(0.93)N on the Si substrate.

The Si substrate was removed through mechanical polishing using diamondparticles and mechanochemical polishing that employs colloidal silicaparticles in an alkaline medium, to thereby obtain the n-typeAl_(0.07)Ga_(0.93)N substrate 1 having an electron density of 3×10¹⁷/cm³and a thickness of 100 μm. Alternatively, the n-type Al_(0.07)Ga_(0.93)Nsubstrate 1 may be obtained by removing an Si substrate throughwet-etching.

A method for producing the component layers of the light-emitting diode100 of the invention will next be described. The light-emitting diode100 was produced through a vapor phase growth method such as, metalorganic vapor phase epitaxy (MOVPE). Employed-gases were ammonia (NH₃),carrier gases (N₂ or H₂), trimethylgallium (Ga(CH₃)₃, hereinafterabbreviated as TMG), trimethylaluminum (Al(CH₃)₃, hereinafterabbreviated as TMA), trimethylindium (In(CH₃)₃, hereinafter abbreviatedas TMI), silane (SiH₄), and cyclopentadienylmagnesium (Mg(C₅H₅)₂,hereinafter abbreviated as CP₂Mg).

Initially, an n-type Al_(0.07)Ga_(0.93)N substrate 1 was placed on asusceptor disposed in a reaction chamber of an MOVPE apparatus, and thetemperature of the substrate was maintained at 1000° C. A carrier gas(10 L/min), ammonia (NH₃) (10 L/min), TMG (100 μmol/min), TMA (5μmol/min), and silane (SiH₄, diluted to 0.86 ppm with hydrogen) (5nmol/min) were fed to the chamber, to thereby form an Si-dopedAl_(0.07)Ga_(0.93)N n-type cladding layer 2 having a thickness ofapproximately 0.5 μm and an electron density of 2×10¹⁸/cm³.Subsequently, a carrier gas (10 L/min), ammonia (NH₃) (10 L/min), andTMG (20 μmol/min) were fed to the chamber, to thereby form a GaN activelayer 3. Subsequently, a carrier gas (10 L/min), ammonia (NH₃) (10L/min), TMG (100 μmol/min), TMA (5 μmol/min), and CP₂Mg (2 μmol/min)were fed into the chamber, to thereby form an Mg-dopedAl_(0.07)Ga_(0.93)N p-type cladding layer 4 having a thickness ofapproximately 0.5 μm. The cladding layer 4 was irradiated with anelectron beam so as to lower electric resistance of the layer, therebycausing a p-type cladding layer 4 to have a hole density of 5×10¹⁷/cm³.Gold (Au) was vapor-deposited on the p-type cladding layer 4 to form apositive electrode, and aluminum (Al) was vapor-deposited on thebackside of the n-type Al_(0.07)Ga_(0.93)N substrate 1 to form anegative electrode.

The thus-produced light-emitting diode 100 exhibited an emission peakwavelength of 365 nm.

The light-emitting diode 100 shown in FIG. 1 has the emission layersandwiched between the positive electrode 5A and the negative electrode5B. Comparing FIGS. 1 and 3, an etching step for exposing the n-typecontact layer 903 included in the conventional light-emitting diode 900can be omitted. As a result, the number of layers to be formed can bereduced. In addition, the metal electrode contact area (ohmic contactarea) of the positive and negative electrodes can be enhanced.

Second Embodiment

A laser diode 200 having a structure shown in FIG. 2 was produced. TheLD 200 contains an n-type Al_(0.07)Ga_(0.93)N substrate 11 having athickness of approximately 100 μm and an electron density of 3×10¹⁷/cm³.

On the n-type Al_(0.07)Ga_(0.93)N substrate 11 were formed, in the ordergiven, an Si-doped Al_(0.07)Ga_(0.93)N n-type cladding layer 12 having athickness of approximately 3 μm and an electron density of 2×10¹⁸/cm³;an Si-doped Al_(0.01)Ga_(0.99)N n-type guide layer 13 having a thicknessof approximately 0.5 μm and an electron density of 5×10¹⁷/cm³; anemission layer 14 of a multiple quantum well (MQW) structure comprisingfive GaN well layers each having a thickness of 2 nm stacked alternatelywith six Al_(0.01)Ga_(0.99)N barrier layers each having a thickness of 5nm; an Mg-doped Al_(0.01)Ga_(0.99)N p-type guide layer 15 having athickness of approximately 0.5 μm and a hole density of 5×10¹⁷/cm³; anMg-doped Al_(0.07)Ga_(0.93)N p-type cladding layer 16 having a thicknessof approximately 1 μm and a hole density of 5×10¹⁷/cm³; and an Mg-dopedGaN p-type contact layer 17 having a thickness of approximately 0.2 μmand a hole density of 7×10¹⁷/cm³. A metal electrode 18A was formed onthe p-type contact layer 17, and a metal electrode 18B was formed on theentire backside of the n-type Al_(0.07)Ga_(0.93)N substrate 11. Thelaser diode 200 of the second embodiment was produced, on the n-typen-type Al_(0.07)Ga_(0.93)N substrate 11 through a metal organic vaporphase epitaxy (MOVPE) method, in a manner similar to that described inthe first embodiment. Specifically, an n-type Al_(0.07)Ga_(0.93)Nsubstrate 11 was placed on a susceptor disposed in a reaction chamber ofan MOVPE apparatus, and the temperature of the substrate was maintainedat 1000° C. A carrier gas (10 L/min), ammonia (NH₃) (10 L/min), TMG (100μmol/min), TMA (5 μmol/min), and silane (SiH₄, diluted to 0.86 ppm withhydrogen) (5 nmol/min) were fed into the chamber, to thereby form anSi-doped Al_(0.07)Ga_(0.93)N n-type cladding layer 12 having a thicknessof approximately 3 μm and an electron density of 2×10¹⁸/cm³.Subsequently, a carrier gas (10 L/min), ammonia (NH₃) (10 L/min), TMG(50 μmol/min), and TMA (1 μmol/min) were fed to the chamber, to therebyform an Si-doped Al_(0.01)Ga_(0.93)N n-type guide layer 13 having, athickness of 0.5 μm and an electron density of 5×10¹⁷/cm³. Subsequently,the emission layer 14 of a multiple quantum well (MQW) structure wasformed. The MQW layer comprises five GaN well layers stacked alternatelywith six Al_(0.01)Ga_(0.99)N barrier layers. Specifically, GaN welllayers each having a thickness of 2 nm are formed by feeding ammonia(NH₃), TMG, and Al_(0.01)Ga_(0.99)N barrier layers each having athickness of 5 nm were formed by feeding ammonia (NH₃), TMG, and TMA.

Subsequently, a carrier gas (10 L/min), ammonia (NH₃) (10 L/min), TMG(50 μmol/min), TMA (1 μmol/min), and CP₂Mg (2 μmol/min) were fed to thechamber, to thereby form an Mg-doped Al_(0.01)Ga_(0.93)N p-type guidelayer 15 having a thickness of approximately 0.5 μm. Subsequently, acarrier gas (10 L/min), ammonia (NH₃) (10 L/min), TMG (50 μmol/min), TMA(5 μmol/min), and CP₂Mg (2 μmol/min) were fed to the chamber, to therebyform an Mg-doped Al_(0.07)Ga_(0.93)N p-type cladding layer 16 having athickness of approximately 1 μm. Subsequently, a carrier gas (10 L/min),ammonia (NH₃) (10 L/min), TMG (100 μmol/min), and CP₂Mg (2 μmol/min)were fed to the chamber, to thereby form an Mg-doped GaN p-type contactlayer 17 having a thickness of approximately 0.2 μm. The p-type contactlayer 17, the p-type cladding layer 16, and the p-type guide layer wereirradiated with an electron beam so as to lower electric resistancethereof, thereby causing the layers to have hole densities of7×10¹⁷/cm³, 5×10¹⁷/cm³, and 5×10¹⁷/cm³, respectively. Gold (Au) wasvapor-deposited on the p-type contact layer 17 to form a positiveelectrode 18A, and aluminum (Al) was vapor-deposited on the backside ofthe n-type Al_(0.07)Ga_(0.93)N substrate 11 to form a negative electrode18B.

In contrast to a conventional laser diode such as the laser diode 950shown in FIG. 4, cracks were not generated in the thus-produced laserdiode 200, despite the n-type cladding layer 12 formed therein having athickness as large as approximately 3 μm. The threshold current of thelaser diode 200 of the present invention is 100 mA, whereas theconventional laser diode 950 has a threshold current of 200 mA. Thereason for the decrease in threshold current is thought to beenhancement in the light confinement due to an absence of cracks. In thelaser diode 200, facets having a mirror surface can readily be attainedthrough cleavage. In addition, electric current can be caused to flowfrom the p-type GaN contact layer 17 to the substrate 11, to therebyomit an n-type GaN contact layer and enhance the metal electrode contactarea (ohmic contact area) of the negative electrode.

In the above-described embodiments, a silicon substrate was employed forforming the Al_(0.07)Ga_(0.93)N substrate through halide epitaxy.However, other substrates, such as silicon carbide (SiC), galliumphosphide (GaP), and sapphire may also be employed. Although in theabove-described embodiments, Group III nitride compound semiconductorlayers are formed on the Al_(0.07)Ga_(0.93)N substrate through metalorganic chemical vapor deposition (MOCVD), other vapor phase growthmethods, such as molecular-beam epitaxy (MBE), halide vapor phaseepitaxy (Halide VPE), and liquid-phase epitaxy (LPE) may also beemployed.

The structure of the light-emitting device is not particularly limited,and the device may have a homo-, hetero-, or double-hetero-structure.These structures may be formed via an MIS junction, a PIN junction, or apn junction. The emission (active) layer may have any quantum wellstructure, for example, a single quantum well (SQW) structure, or amultiple quantum well (MQW) structure which comprises well layers andbarrier layers having a band gap higher than that of a well layer.

In the present invention, Group III Elements in the Group III nitridecompound semiconductors may be partially substituted by boron (B) orthallium (Tl), and nitrogen atoms may be partially substituted byphosphorus (P), arsenic (As), antimony (Sb), or bismuth (Bi). When thesesemiconductors are used in light-emitting devices, 2-component or3-component Group III nitride semiconductors are preferred.

In the first and second embodiments of the invention, the backside ofthe substrate is formed from a metallic negative electrode 5B or 18B.The metal electrode may further be coated with another layer. Forexample, in a light-emitting diode, a light-reflecting layer may beformed on the backside metal layer in order to enhance light extractionefficiency. Examples of preferred metals for forming thelight-reflecting layer include Al, In, Cu, Ag, Pt, Ir, Pd, Rh, W, Mo,Ti, and Ni. These metals may be used singularly or in combination of twoor more species, for instance, in the form of an alloy. The metalliclight-reflecting layer having the same construction as above may bedisposed on the positive electrode side; for example, in alight-emitting diode.

In the above embodiments, the Al_(0.07)Ga_(0.93)N conductive substratewas obtained without doping. However, an n-type Al_(x)Ga_(1−x)Nsubstrate having a controlled conductivity may be formed by using silane(SiH₄) to dope silicon (Si), and a Group III nitride compoundsemiconductor device may be formed thereon. In the embodiments describedabove, the compositional proportion of aluminum contained in thesubstrate 1 or 11; the cladding layer 2, 4, 12, or 16; the guide layer13 or 15; or the barrier layer in the MQW represents a typical example.Thus, any material that satisfies the formula Al_(x)Ga_(1−x)N (0<x<1)may be employed. In such a case, the proportion ‘x’ may be variedaccordingly in the components semiconductor layers.

In addition, the light-emitting devices of the above embodiments werefabricated such that electricity passes through the Al_(x)Ga_(1−x)Nsubstrate. Alternatively, both the positive electrode and the negativeelectrode may be disposed on the top surface of the various devicelayers. Since one of the important features of the invention isemployment of an Al_(x)Ga_(1−x)N substrate in which ‘x’ is approximatelyequal to ‘x’ in a Group III nitride compound semiconductor layercharacterized by having a high value of ‘x’, the positions for disposingelectrodes are not limited.

While the invention has been described in terms of a certain preferredembodiment, other embodiments apparent to those of ordinary skill in theare also within the scope of this invention. Accordingly, the scope ofthe invention is intended to be defined only by the claims that follow.

What is claimed is:
 1. A Group III nitride compound semiconductor devicecomprising: a substrate made of aluminum gallium nitride satisfying theformula (Al_(x)Ga_(1−x)N, 0<x<1), one or more Group III nitride compoundsemiconductor layers formed on a first surface of the substrate, a metalelectrode formed on a second surface of said substrate, and a claddinglayer comprising aluminum gallium nitride Al_(x)Ga_(1−x)N (0<x<1).
 2. AGroup III nitride compound semiconductor device according to claim 1,wherein: composition ratio ‘x’ of said cladding layer Al_(x)Ga_(1−x)N isequal or nearly equal to composition ratio ‘x’ of said substrateAl_(x)Ga_(1−x)N.
 3. A Group III nitride compound semiconductor deviceaccording to claim 2, wherein said cladding layer is stacked on thefirst surface of the substrate and has a thickness of 1-20 μm.
 4. AGroup III nitride compound semiconductor device according to claim 1,wherein among the Group III nitride compound semiconductor layersstacked on the first surface of the substrate, all layers having athickness of more than 10 nm are independently formed of a compoundrepresented by Al_(x)Ga_(1−x)N, wherein 0≦x≦1.
 5. A Group III nitridecompound semiconductor device according to claim 1, wherein each of theGroup III nitride compound semiconductor layers stacked on the firstsurface of the substrate is formed of a compound represented byAl_(x)Ga_(1−x)N, wherein 0≦x≦1.
 6. A Group III nitride compoundsemiconductor device according to claim 1, wherein said cladding layeris stacked on the first surface of the substrate and has a thickness of1-20 μm.
 7. A Group III nitride compound semiconductor devicecomprising: a substrate made of aluminum gallium nitride satisfying theformula (Al_(x)Ga_(1−x)N, 0<x<1), one or more Group III nitride compoundsemiconductor layers formed on a first surface of the substrate, a metalelectrode formed on a second surface of said substrate, and a firstlayer comprising Al_(x)Ga_(1−x)N (0<x<1) stacked on the first surface ofthe substrate and having a thickness of 1-20 μm.